The present invention relates to the lubrication of high speed vertical shafts, such as vertical drill spindles.
Drills for drilling holes in workpieces, such as metal workpieces, are conventionally mounted on a movable carriage or deck so as to be capable of movement toward and away from the workpiece. Rotatably carried by the carriage is a shaft to the front end of which a drill bit is fastened. The spindle is driven at a relatively high speed, i.e., upwards of 500 rpm, by suitable gearing. Upper and lower ends of the shaft are rotatably mounted by bearings, e.g., a ball bearing at the lower end and a roller bearing at the upper end.
In order to cool the shaft and cool and lubricate the bearings during a drilling operation, it is conventional to conduct a continuous flow of lubricating oil vertically along the outer periphery of the shaft from one bearing to another. This cooling function is necessary to prevent excessive thermal expansion of the shaft and bearings. The shaft is mounted within the bore of a vertical stationary sleeve, wherein an annular gap formed between the outer periphery of the shaft and the inner wall of the bore forms a vertical channel for the oil.
The oil is supplied from a reservoir by a pump which introduces the oil into the top of the oil channel whereupon the oil circulates through the upper bearing, downwardly through the channel, through the lower bearing, and then back to the reservoir.
The shortcomings of such a lubricating system, aside from the expense of having to provide a pump, include the need to provide a separate monitor to indicate that the pump is functioning to pump oil. In the absence of such a monitor there would occur a risk that the shaft and bearings could become overheated and damaged if the pump malfunctioned. Furthermore, the frictionally meshing toothed wheels of the pump become hot due to frictional contact and thus impart heat to the oil. This heating of the oil diminishes the ability of the oil to cool the shaft and bearings.
Another type of lubricating system has been proposed which involves spraying a mist of lubricating oil into each bearing. However, such a system is expensive, and the oil mist does no always adequately cool the spindle and bearings.
A further problem which may be encountered in connection with relatively rapidly rotating shafts (e.g., shafts rotating at least at 500 rpm) relates to a tendency for the shaft to vibrate. The unsupported segment of the shaft extending between the bearings is substantially longer than the combined segments of the shaft supported within the bearings. If the load applied to the shaft during drilling results in the unsupported segment becoming bent (i.e., becoming non-symmetrical about the axis of rotation), vibration of the shaft will occur due to the high speed of rotation. Neither of the afore-mentioned lubricating systems can appreciably reduce such vibration.